Thermodynamic heat engines may be used to convert heat energy to mechanical work. A thermodynamic heat engine relies upon a (relatively) high temperature heat source and a (relatively) low temperature heat sink in order to operate according to a thermodynamic cycle. In general, the greater the temperature difference between the heat source and the heat sink, the higher the efficiency of the thermodynamic heat engine.
A closed cycle thermodynamic heat engine is a thermodynamic heat engine in which a fixed amount of a working fluid is contained within the heat engine, which working fluid is repeatedly subjected to repetitions of the thermodynamic cycle. A gas phase closed cycle thermodynamic heat engine is a closed cycle thermodynamic heat engine in which the working fluid is maintained in a gaseous state throughout the thermodynamic cycle.
A typical thermodynamic cycle for a closed cycle thermodynamic heat engine includes cooling, compression, heating and expansion as basic processes. Non-limiting examples of closed cycle thermodynamic heat engines include those which operate according to the Carnot cycle, the Stirling cycle and the Ericsson cycle.
The Carnot cycle is characterized by a sequence of processes performed on the working fluid which consist of isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression.
The Stirling cycle is characterized by a sequence of processes performed on the working fluid which consist of isothermal expansion, cooling at constant volume, isothermal compression, and heating at constant volume.
The Ericsson cycle is characterized by a sequence of processes performed on the working fluid which consist of isothermal expansion, cooling at constant pressure, isothermal compression, and heating at constant pressure.
The efficiency of some closed cycle thermodynamic heat engines can be increased through the use of a regenerator. A regenerator is a heat storage device and secondary heat exchanger which may be interposed between the cooling and heating processes of the heat engine in order to exchange heat with the working fluid and thus retain heat within the system which could otherwise be lost to the environment exterior to the heat engine. Thermodynamic heat engines operating according to the Stirling cycle and the Ericsson cycle frequently include regenerators in order to increase their efficiencies.
The temperature within the earth below the frost line (i.e., about 2 meters below the surface of the earth in Northern countries such as Canada) remains relatively stable throughout the year. An estimate of the average stable year-round ground temperature in Canada at a point within about 1 meter below the frost line is about 5-10 degrees Celsius.
The temperature within the earth below the frost line tends to increase as the distance below the surface (i.e., depth) increases, particularly within the earth's crust. This increase in temperature as a function of depth is referred to as the “geothermal gradient”. An estimate of the average geothermal gradient throughout the earth's crust is about thirty degrees Celsius per kilometer (30° C./km).
The temperature within the earth at a location two kilometers below the earth's surface may therefore be about 50-55 degrees Celsius higher than the temperature several meters below the earth's surface.
As a result, the stability of the ground temperature below the frost line and/or the geothermal gradient can potentially be utilized for the operation of a heat engine, such as a closed cycle thermodynamic heat engine.
Geothermal or ground source heat pump systems are known for houses and other buildings. Such systems typically include a heat exchanger located within the building for transferring heat to and from a closed loop containing water or antifreeze as a working fluid, which closed loop extends between the heat exchanger and a location in the ground which is relatively shallow but is below the frost line. The working fluid is typically circulated through the closed loop by a pump. Geothermal or ground source heat pump systems may be used to transfer heat from the ground to the building (i.e., for heating of the building) or to transfer heat from the building to the ground (i.e., for air conditioning of the building).
U.S. Patent Application Publication No. US 2007/0245729 A1 (Mickleson) describes a geothermal energy system comprising an injection borehole, a production borehole, a heat extraction system located at the earth's surface and coupled to the production borehole, piping coupled to and extending between the heat extraction system and the injection borehole, and a spanning borehole portion extending from the injection borehole to the production borehole and extending through hot rock. The heat extraction system may be comprised of a turbine, an exchanger-piping system or a Stirling engine and alternator combination. A geo-fluid is circulated through the geothermal energy system in order to mine heat from hot rock resources and deliver the heat to the heat extraction system.
U.S. Patent Application Publication No. US 2008/0209904 A1 (Sumrall) describes an electrical power generating system including a Stirling engine and a high temperature source coupled to a hot chamber of the Stirling engine, wherein the high temperature source comprises heat from below the earth's surface. In an embodiment of the system, the high temperature source may be a borehole such as a dry hole, an oil well, or a gas well. The low temperature source for the Stirling engine may be comprised of a body of water, in which case the Stirling engine is thermally coupled to the body of water. In some embodiments, the Stirling engine may be located in the body of water.
U.S. Patent Application Publication No. US 2008/0223032 A1 (Sumrall) describes an electrical power generation system similar to that described in U.S. Patent Application Publication No. US 2008/0209904 A1 (Sumrall). The electrical power generation system includes a power generating means comprising a hot junction and a cold junction, wherein the power generating means may be comprised of a thermoelectric generator or alternate power generating means including Stirling engines, Rankin engines, Matteran energy cycle engines, flash power plants, dry steam power plants, binary power plants, flash/binary combined cycles, and the like.